When drilling deep bore holes in the earth, sections of the bore hole can cause drag or excess friction which may hinder weight transfer to the drill bit, or cause erratic torque in the drill string. These effects may have the result of slowing down the rate of penetration, creating bore hole deviation issues, or even damaging drill string components.
Friction tools are often used to overcome these problems by vibrating a portion of the drill string to mitigate the effect of friction or hole drag. These friction tools form part of the downhole assembly of the drilling string, and can be driven by the variations in the pressure of drilling fluid (which may be air or liquid, such as drilling mud) flowing through the friction tool. Accordingly, the operation or effectiveness of a friction tool—namely, the frequency of vibrations generated by the friction tool—may be affected by the flow rate of drilling fluid pumped through the string. Controlling the frequency of vibration thus may involve varying the flow rate of the drilling fluid at the surface, and ceasing operation of the friction tool may require cutting off the flow of drilling fluid at the surface. Varying or cutting off the drilling fluid flow, however, may impact the operation of other components in the drilling string.
Furthermore, it is not always desirable to run a friction tool during the entirety of a drilling operation. For instance, it may be unnecessary or undesirable to run the tool while the drill bit is at a shallow depth, or at other stages of the drilling operation where the added vibration of the friction tool is problematic. During those stages, the drill string may be assembled without the friction tool. However, when a location in the bore hole is reached where the need for a friction tool is evident, it is then necessary to pull the downhole assembly to the surface to reassemble the drilling string to include the friction tool, then return the drilling string to the drill point. This process can consume several work hours.